PSTN: Outside Plant

The original premise behind the telephone exchange or Central Office (CO) was to run only one wire or set of wires into each house and have a centrally located facility for switching connections via operator (or automated equipment). Even though new homes today may see six or more wire pairs, plus a coaxial cable for broadband cable television, the basic principle remains the same: each line to the customer forms a loop that passes through to the CO.

The collection of cabling and facilities that support all local loops outside the CO (or “wire center”) is known as the “loop distribution plant” and is owned by the Local Exchange Carrier (LEC). It starts out from the CO in a large underground cable vault with primary feeder cable (F1) to reach out over copper (or fiber) to the Serving Area Interface (SAI) for that area (look for a large grey or green box with doors mounted on a concrete pedestal in most areas of the United States). F1 cable is typically 600 to 2000 or more pairs and usually must be buried because of its weight (although fiber-optic F1 cable can be aerial if needed). It often is armored or pressurized and generally is enclosed in a concrete trench all the way to the CO, with manholes or other access points at least every 750 feet to allow for installation of repeaters (for digital trunks like the T1), loading coils, and other necessary equipment. In most of the world, the LEC is able to keep F1 and SAI fairly secure through physical locks, alarms, and so on.

At the SAI, F1 feeds are cross-connected to secondary feeder cable (F2) that goes out over copper underground to pedestal boxes where the distribution cable is split out or on poles to aerial drop splitters. Subscriber drop wires are then cross-connected to the F2 at that point. In rural areas, even lower-level cable facilities (F3, F4, F5) may exist before a drop wire is terminated. A box is installed where the drop wire is terminated outside the subscriber’s premises and this box is considered the demarcation point for the LEC. All wiring from there to the CO is the responsibility of the LEC, and from there to the phone devices themselves is the subscriber’s responsibility (or that of the landlord). Physical security of that inside wiring—particularly in shared facilities—can be an issue in some cases. And F2 or lower feeds and pedestals are not well secured in general (and present the biggest opportunity to an eavesdropper).

Where growth or other planning challenges have exhausted the supply of F1 or F2 pairs, it’s sometimes necessary for the LEC to install Remote Terminal (RT) equipment (sometimes called “pair gain” systems) that can multiplex multiple local loops on to a digital T-carrier (using Time-Division Multiplexing (TDM) over a 4-wire copper or pair of fiber-optic cables), or via older Frequency-Division Multiplexing (FDM) systems. RT units generally are locked and alarmed, however. And it is much more difficult to eavesdrop on a digital trunk (such as a T-carrier) or FDM system because of the costly equipment required. Figure 1 shows a diagram of a central office equipped with outside distribution plant (ODP).

Figure 1: The Central Office with ODP*

* This classic example assumes no fiber is in use to these SAIs within the CO (see SONET example in Figure 2).

In addition to the loop distribution plant, the LEC will have outside plant for trunking between central offices, and the LEC and other Inter-exchange Carriers (IXCs) will have outside plant for long distance connections between COs and other switching centers such as toll centers. And the LEC or other Competitive Local Exchange Carriers (CLECs) may run fiber for SONET (or SDH) rings (see Figure 2).

Figure 2: A Modern SONET Ring Example

The diagram in Figure 2 shows that by using path diversity for fiber-optic routes along with SONET rings with Add-Drop Multiplexers, several self-healing SONET rings provide F1 and some F2 subscriber loop feeds as well as trunking between two central offices. Large business customers can also connect to this SONET ring for high-capacity voice and data services if they are located close enough to the buried fiber.